Self orienting micro plates of thermally conducting material as component in thermal paste or adhesive adhesive

ABSTRACT

The present invention relates generally to thermally-conductive pastes for use with integrated circuits, and particularly, but not by way of limitation, to self-orienting microplates of graphite.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending application Ser. No.12/039,309, filed Feb. 28, 2008, allowed, which is a continuation ofU.S. Pat. No. 7,351,360, filed Nov. 12, 2004, the entire contents ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to thermally-conductive pastesfor use with integrated circuits, and particularly, but not by way oflimitation, to self-orienting microplates of graphite.

BACKGROUND

The information provided below is not admitted to be prior art to thepresent invention, but is provided solely to assist the understanding ofthe reader.

Computer chips operate at ever greater power densities as circuits arepacked more and more closely together. This higher power density can notbe effectively accommodated by traditional methods of cooling. Higherpower consumption without concomitant heat removal leads to higher chipoperating temperatures which can result in severe reliability,performance, and lifetime concerns. FIG. 1 shows a schematic of aconventional cooling solution comprising a chip mounted on a ceramicmodule and a heat transfer paste applied to the top surface of the chipand squeezed against a heat sink. This creates a sandwich of a thermalpaste between the chip and the heat sink. Heat sinks typically have finstructures that transfer heat, generated by the chip, to air, blownacross the fins by a fan. The rate of heat transfer of such coolingdesigns is limited by the thermal conductivity of the gap fillingmaterial. There exists a need to provide heat-transfer pastes havingimproved thermal conductivity properties.

SUMMARY OF INVENTION

The present invention provides platelet and/or disk shaped particles ofa conducting material as a component of a thermal paste to maximize heattransfer from one particle to another. An aspect of the presentinvention provides a conductive material having anisotropic thermalconductive properties. An aspect of the present invention provides asubstantially planar heat source and a substantially planar heat sink.An aspect of the present invention provides a thermal conductive pastedisposed between the heat source and heat sink wherein the platelets areoriented with the plane of their major surfaces substantially parallelto the plane of the heat source and sink. A further aspect provides thatwhen the platelets are thus oriented, the direction of maximum heattransfer rate is perpendicular to the plane of the heat source andperpendicular to the plane of the heat sink.

An aspect of the present invention provides platelets comprised oforiented graphite. A further aspect of the present invention providesgraphite platelets wherein the graphite basal plane (L e. the directionof maximal heat transfer) is perpendicular to the plane of the platelet.

An aspect of the present invention provides a paste comprisingthermally-anisotropic platelets suspended in a liquid. A further aspectof the present invention provides a liquid component which may be an oilor a liquid metal. In an aspect, the oil comprises a polyalphaolefin(PAO) oil. In an aspect, the liquid metal may be a gallium/indiumeutectic or a gallium/indium/tin eutectic. In a further aspect, theliquid metal may be a solder.

A further aspect provides the conductive platelets are coated with athin layer of a metal or other conductive material. In an aspect, thecoating serves to strengthen the platelets against fracture. In anaspect, the coating serves to make the platelets more wettable by aliquid matrix. In an aspect, the coating serves to make the plateletsurface more compatible with a solder matrix both in terms of wettingand fusing. According to an aspect of the present invention, the coatingmay be a thin layer of a metal. According to a preferred, but notlimiting, aspect, the metal may be copper, gold, or nickel. According toa further aspect, the surface of the platelets may be provided any of anumber of commonly used surface treatments, such as, but not limited to:plasma oxidation, plasma sulfonation, ozone treatments in the presenceor absence of ultraviolet light, alkaline surface hydrolysis,silanation, (treatment with any number of silane containing substancessuch as hexamethyl silyl disilizane, or trimethoxysilylpropyl amine),and plasma ammonia treatment. According to a further aspect, the coatingmay be deposited by any conventional method. Suitable methods include,but are not limited to, CVD of powders suspended in fluidized bedreactors, electroless or electroplating, dipping and drying, plasmaprocessing, and spraying.

According to an aspect, the present invention provides a pastecomposition having anisotropic thermal transmission characteristicscomprising: platelets of a material having a basal plane in az-direction with a first length and a platelet plane having second andthird lengths in an x- and a y-direction respectively, said second andthird lengths larger than said first length; and a liquid. According toan aspect the basal plane defines the direction of maximal thermalconductivity.

According to an aspect, the present invention provides a pastecomposition, wherein the platelets comprise a graphite selected from thegroup consisting of single crystal graphite, multiwall carbon nanotubes,oriented pyrrolytic graphite fibers, and rolled grapheme films.According to a further aspect, the graphite is isotopically-enriched tohave a content of ¹²C above the natural abundance. According to afurther aspect, the graphite consists essentially of isotopically pure¹²C. According to a further aspect, the graphite isisotopically-enriched to have a content of ¹³C above the naturalabundance. According to a further aspect, the graphite consistsessentially of isotopically pure ¹³C.

Still other aspects and advantages of the present invention will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described preferred embodiments ofthe invention, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects,without departing from the invention. Accordingly, the description is tobe regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 is a schematic of a chip mounted on a ceramic module, a heattransfer paste applied to the top surface of the chip, and squeezedagainst a heat sink;

FIG. 2 depicts the crystal structure of graphite;

FIG. 3A depicts a boule of single-crystal graphite, the basal planes ofwhich are in the x-y plane of the drawing;

FIG. 3B depicts platelet sections sliced from a boule; the graphitebasal plane is oriented perpendicular to the plane of the section;

FIG. 4 depicts a platelet section of a boule and smaller pieces derivedtherefrom;

FIG. 5 depicts cylindrical, oriented graphite, either graphite fiber ormultiwalled carbon nanotube (MWNT), and platelet sections sliced from anend thereof;

FIG. 6 depicts a plurality of M′WNT bundled for ease of manipulation andthe corresponding plurality of sliced from an end of the bundle;

FIG. 7 depicts randomly oriented MWNT plates formed into a pastes andapplied in an oriented manner in the gap between a chip heat source anda heat sink;

FIG. 8 depicts a stack comprising a plurality of graphite platelet, eachrotated 90° with respect to an adjacent platelet;

FIG. 9 depicts a bare graphite platelet, a fiber or MWNT plate, beforeand after coating with a material; and

FIG. 10 is an optical micrograph of graphite plates mixed with epoxy.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The term “graphitic” comprehends pure graphite, as well as material thatis “substantially graphite,” as well a material having graphitecharacter.

Reference is made to the figures to illustrate selected embodiments andpreferred modes of carrying out the invention. It is to be understoodthat the invention is not hereby limited to those aspects depicted inthe figures.

Graphite is anisotropic with respect to thermal conductivity. Thethermal conductivity in the basal plane of graphite (the so calledgraphitic plane) is extraordinarily high.

Typically, the conductivity parallel to the basal plane is about 2000W/rn-K. These values are comparable to those of diamond, the best-knownthermal conductor. However, values for the conductivity perpendicular tothe graphitic plane are typically about two orders of magnitude lower,or roughly 10 W/m-K. FIG. 2 illustrates the basal plane of graphite. Thebasal plane is defined by the plane of the C-C covalent bonds.

Thermal pastes comprising randomly-oriented, ground pyrollitic graphitepowders having an average thermal conductivity of about 100 W/m-K areconventional to the art. A conductivity of 100 W/m-K is superior to thatof most materials. However, the present invention represents animprovement over the art by providing particles oriented such that thebasal planes of the particles are aligned in the direction of thedesired heat flow (normal to the chip surface). Thus the conductivity ofthe paste is dramatically improved. The invention is not limited tographite, but may suitably be practiced with any thermally anisotropicmaterial. Any crystalline substance will have some anisotropy. Theinvention is advantageously practiced with a crystal, such as graphite,having covalency restricted to a single plane.

The present invention optimizes the thermal conductivity of graphitewhile maintaining particle sizes appropriate for pastes, by providinggraphite particles which have a platelet, or plate-like, shape and wherethe graphitic plane is oriented perpendicular to the plane of theplatelet. If the aspect ratio of the platelet were optimal, thencomposites made from such material would have a preponderance ofplatelets stacked, or self oriented, in this preferred orientation. Theabsolute size of these particles can vary for different purposes.However, a platelet shape is always desired for processing purposes, andis defined by an optimum aspect ratio (platelet flatness), which is theratio of platelet thickness to the two length dimensions.

There are competing motivations for keeping the particles large or smallon an absolute size basis. The larger they are, the fewer the number ofinterfaces that must be crossed in a given thermal path. This improvesconductivity, as an interface nearly always impedes thermal transport.The smaller they are, the more particle-like the platelets become.Pastes made from smaller particles are better able to conform tonon-uniformities in the gap between the heat source and the heat sink.The more the paste conforms to the gap between the source and sink, thebetter will be

Graphite is anisotropic with respect to thermal conductivity. Thethermal conductivity in the basal plane of graphite (the so calledgraphitic plane) is extraordinarily high. Typically, the conductivityparallel to the basal plane is about 2000 W/rn-K. These values arecomparable to those of diamond, the best-known thermal conductor.However, values for the conductivity perpendicular to the graphiticplane are typically about two orders of magnitude lower, or roughly 10W/m-K. FIG. 2 illustrates the basal plane of graphite. The basal planeis defined by the plane of the C-C covalent bonds.

Thermal pastes comprising randomly-oriented, ground pyrollitic graphitepowders having an average thermal conductivity of about 100 W/m-K areconventional to the art. A conductivity of 100 W/m-K is superior to thatof most materials. However, the present invention represents animprovement over the art by providing particles oriented such that thebasal planes of the particles are aligned in the direction of thedesired heat flow (normal to the chip surface). Thus the conductivity ofthe paste is dramatically improved. The invention is not limited tographite, but may suitably be practiced with any thermally anisotropicmaterial. Any crystalline substance will have some anisotropy. Theinvention is advantageously practiced with a crystal, such as graphite,having covalency restricted to a single plane.

The present invention optimizes the thermal conductivity of graphitewhile maintaining particle sizes appropriate for pastes, by providinggraphite particles which have a platelet, or plate-like, shape and wherethe graphitic plane is oriented perpendicular to the plane of theplatelet. If the aspect ratio of the platelet were optimal, thencomposites made from such material would have a preponderance ofplatelets stacked, or self oriented, in this preferred orientation. Theabsolute size of these particles can vary for different purposes.However, a platelet shape is always desired for processing purposes, andis defined by an optimum aspect ratio (platelet flatness), which is theratio of platelet thickness to the two length dimensions.

There are competing motivations for keeping the particles large or smallon an absolute size basis. The larger they are, the fewer the number ofinterfaces that must be crossed in a given thermal path. This improvesconductivity, as an interface nearly always impedes thermal transport.The smaller they are, the more particle-like the platelets become.Pastes made from smaller particles are better able to conform tonon-uniformities in the gap between the heat source and the heat sink.The more the paste conforms to the gap between the source and sink, thebetter will be the thermal conductivity. However, a smaller particlesize implies a higher number of interfaces with a concomitant thermalperformance penalty.

At the extreme upper size extreme, a single sheet of material can beplaced between the source and sink; thus, eliminating all but twointerfaces. The single sheet would have virtually no ability to conformto irregularities in the shape of the gap. Thermal transfer would beimpeded where the gap remained unfilled. At the lower size extreme,nano-sized particles can be utilized, as long as they have an aspectratio (thickness (z) to plate length (x and y)) such that they orientwith the plane of the plate parallel to the plane of the source and/orsink. A plate of the present invention positioned parallel to the heatsource will have its basal plane (the direction of maximum rate of heattransfer) perpendicular to the source and parallel to the direction ofheat transfer. Typically, the particle size will be in the micron to mmsize range, permitting a reasonable balance of these competingproperties—as well as cost and convenience.

One approach for larger particles is to cut or slice graphite intosheets from a single crystal boule. In these sheets the graphitic planemust be perpendicular to the plane of the sheet. These can easily be cutto about 10 mil thicknesses by sawing with a diamond blade. FIG. 3illustrates this where slices are sawed off making platelets. In thefigure, the basal (graphitic) plane is shown in the boule to be in thex-y plane of the drawing. After the cut, the platelet is unstable ingravitational or shear force fields and will tip over to lie in a stableposition relative to a flat surface. This orients the basal plane of thegraphite in the vertical direction—or stated another way perpendicularto the flat surface. So oriented, the high conductivity plane isparallel to the direction of heat flow from the heat source to the heatsink.

Often, particles smaller than the boule cross sectional dimension aredesired. Graphite sheets may be reduced in size to platelets by any of avariety means. Graphite particles of the present invention havesubstantially parallel and substantially planar major surfaces. Eachmajor surface has x and y dimension, each larger than the thickness ofthe platelet. Each platelet is characterized by an aspect ratio—theratio of the thickness to any of the x or y dimensions. Preferably, eachof the x and y dimensions is no smaller than about 5 times the thicknessto ensure that the particles will lie flat. The aspect ratio may be aslow as 2:1. Though this will be dependent on the details of processingsuch as how much sheer force is applied during paste application, howmuch downward force, etc. If a 5:1 aspect ratio is used for illustrativepurposes, a 10 mil thick sheet may be ground, or otherwise reduced,until the average particle size was about 50 mil on a side. Theresulting powder constitutes the solid component of the thermal paste.

A single crystal graphite block of graphite is shaved or cutperpendicular to the basal plane in thin sheets. As shown in FIG. 3. Inan embodiment, 10 mil (1 mil=0.0254 millimeter; =0.001 inch) slices maybe made with a diamond saw. In a preferred embodiment, 10 micron slicesmay be obtained using an automatic microtome which can easily make 10micron thick cuts and advance the block of source material for each newcut up to thousands of cuts.

Thin sheets cut in that way are then ground until either the x or y sizeis some multiple of the thickness (such as 50 micron for this examplefor a 10 micron thickness) to assure that the resulting pieces will liefiat with the basal orientation perpendicular to the substrate surface.Since the sliced sheets of graphite will tend to break between the basalplanes preferentially (as illustrated in FIG. 4) needle shapes wouldeventually result. If we designate the needle direction the x direction,and the thickness the z direction, then we need only to make sure togrind until the y direction is no smaller than 50 micron. (i.e., the ydirection should be larger than the z direction to make sure the needleor platelet lies such that the basal graphite plane is parallel to thedesired direction of heat transfer). This condition is shown in FIG. 4b. If they dimension becomes comparable to or smaller than the zdirection, as in FIG. 4 c, then the needle would be prone to reorientingto lie on the wrong side, resulting in loss of thermal conductivity, asshown in FIG. 4D.) In FIG. 4, an ideal target level of grinding would bestage B, where the platelets are as small as they can be while remainingstable in the preferred orientation.

In an embodiment, oriented, pyrolitic graphite fibers (i.e. carbonfibers), instead of single crystal graphite, provide the source materialthe graphite powder. Carbon fibers confer a benefit in being far lessexpensive than single crystal graphite. In this process, graphite fibersas shown in FIG. 5 or bundles of them, as shown in FIG. 6, whichtypically have the basal plane oriented in the fiber direction, aresliced by a microtome in a process similar to that for processing ofsingle crystal graphite. Where the slice thickness is significantlysmaller than the fiber diameter, the platelet form factor, or disk,results and no further sizing is required. This has the strong advantageof requiring no further grinding and its associated end-point targeting.

In a further embodiment, the fibers may be bundled and bound with athermal conductive polymer or metal prior to cross-sectioning. Thiseases handling issues and provides a single heat spreader the size ofthe chip that can be attached through the use of a thermally conductivepolymer, solder, or liquid metal.

In an alternative embodiment, multiwall carbon nanotubes (MWNT) serve asthe source material. Carbon nanotubes, as do carbon fibers, have theequivalent of a graphitic-plane running in the axial direction. Bundlesof multiwalled nanotubes are sectioned perpendicularly to the axialdirection. The ratio of the diameter to the thickness of the slice ischosen to yield the desired aspect ratio, causing the vast majority ofthe slices to orient with the high conductivity direction being in linewith the desired thermal path. Oriented carbon fibers and MWNT havethermal conductivities of about 1000 W/in K, about a factor of 2 lowerthan for single crystal graphite. However, these carbon forms providehandling advantages that may outweigh the lower conductivity. FIG. 7illustrates schematically the thermal interface material made from thesedisks mixed with a liquid medium and applied between the heat source andheat sink.

Graphene films may be the source of oriented graphite. The preparationof graphene from highly oriented pyrolytic graphite has been reported inScience [306 Science 666 (2004)]. Rolling graphene into a cylinder andtaking transverse sections will properly orient the basal plane.

In an embodiment, the graphite is enriched in ¹²C. In an embodiment, thegraphite comprises isotopically pure ¹²C. It is known that theconductivity of diamond increases with isotopic purity as the percentageof ¹³C is reduced. Or, conversely, graphite could be made fromisotopically purified ¹³C for similar advantage.

Various sectioning methods may be advantageous, depending on thethickness. For the thickest sections, a diamond saw may be used. Forsections approximating 10 microns, a microtome may be suitably employed.Lasers may be used to slice graphite and to cut it into appropriatelyshaped platelets. The cutting of graphite into slices and into x-yshapes and sizes could be facilitated by photolithography, laserablation, or dicing tools. While very expensive, it would provide highdegree of control.

In an embodiment, a chip dicing tool is used to cut shapes on the orderof a millimeter on a side from the sliced sheets. Providing preciseshapes optimizes packing efficiency. For instance, triangles cut fromsingle crystal graphite sheets will pack with fewer voids than will theneedle shapes that result from simple grinding. Suitable shapes include,but are not limited to, squares, hexagons, and other polygons.

In an embodiment, graphite powders, suitably shaped into platelet and/ordisk shapes are coated with a metal or other material, as shown in FIG.9. Coating the particles confers several advantages, including: a)strengthening the platelets to prevent further cleavage; b) in makingthe surface more wettable to the liquid matrix; c) making the surfacemore compatible with a solder matrix both in terms of wetting andfusing; and d) making the surface optimum for the surfactant and mediumof liquid suspensions. Such a coating may advantageously be a thin layerof a metal, such as, but not limited to, copper, gold, nickel, solder,and low melting metals. Such a coating may also comprise any commonlyused surfaces and surface treatments such as, but not limited to plasmaoxidation, plasma sulfonation, ozone treatments in the presence orabsence of ultraviolet light, alkaline surface hydrolysis, silanation,(treatment with any number of silane containing substances such ashexamethyl silyl disilizane, or trimethoxysilylpropyl amine), and plasmaammonia treatment. Copper surfaces may advantageously be treated withbenzotriazole (BTA). Silver and gold surfaces may benefit from treatmentwith an alkane thiol. Silicon, silica, or oxide surfaces may be treatedby silanation such as treating with trimethoxysilylpropyl amine. Thiscoating could be deposited by such methods as CVD while the powders aresuspended in a fluidized bed reactor, by electroless or electroplating,by dipping and drying, by plasma processing, or by other suitable means.

In an embodiment, the platelets are coated with a metal. The metal maybe an alloy or multiple layers of metal. Multiple layers of dissimilarmetals may be beneficial especially if they are precursors to an alloysuch as a tin coating over a copper coating over the graphite plate.This is because once these are in place between the heat source and heatsink, they may be heated once above the melting point of the lowestmelting component metal which will then cause alloy formation andsolidification if the alloy melt point is higher. This will fuseparticles. In an alternative embodiment, a portion of the particles,preferably one halt are coated with one constituent metal of an alloy,for example tin, and another portion (in this example, the other half)of the particles are coated with the other constituent metal of analloy, in this example, copper. The particles are randomly mixed in thepaste, and applied to the gap between the heat source and heat sink, andany solvent is removed by drying. Upon heating above the temperature ofthe lowest melting point of these two metals, alloy formation willensue, and form a bond between the majority of particles providing asolid mass.

Where single crystal graphite sliced are used (as shown in FIG. 3),there is an advantage to stacking the slices as shown in FIG. 8. Thestacked slices may have alternating or random angular orientations. Thestacked slices are adhered, preferably, but not limited to a materialhaving a high thermal conductivity. This aligns the basal planes in thez-direction and produces a multilayered composite with much greaterresistance to cracking. This resistance to cracking is the result of thebasal plane being oriented always in the z-direction always but randomlyin the x and y directions. So, crack propagation would be arrested ateach stacked layer. This would allow grinding to the desired averageparticle size in the x-y dimension without the formation of needlestructures. Elimination of needle structures also confers rheologicalperformance benefits. In an embodiment, single layers of graphite arestacked in alternating or random directions between thin metal sheetsand laminated. The sheets may comprise any metal, such as, but notlimited to, copper, gold, or nickel.

In a preferred embodiment, the graphite powder materials are mixed witha liquid medium to form a paste. In an embodiment, the liquid comprisesa liquid metal such as gallium. In an embodiment, the liquid metalcomprises a gallium/indium eutectic. In an embodiment, the liquid metalcomprises a gallium/indium/tin eutectic. In an embodiment, the liquid isan oil. In an embodiment the liquid is any of PAO oils, low melt waxes,oils, solvents, and adhesives. A preferred oil is a polyalphaolefin(PAO) oil.

Such a paste would then be applied in between the heat source (chip) andthe heat sink to facilitate heat transfer. Paste processing and transferto the chip can be done using an optimized highly concentratedsuspension of platelets. Transfer can be accomplished by shearing thepaste in such a way that favorable arrangement of the particles occurs.The shear and subsequent particle alignment of minimized particleinteractions has the tendency to form 2-D rather than 3-D structures orin other words to align the constituent particles such that their flatfaces were flat against all mating surfaces which would maximize theorientation of the basis planes to each other and to the planes of thechip and heatsink. The phenomenon is observable though shear thinningbehavior of the paste, or a decrease in viscosity with increasing shearrate.

In an alternative embodiment, a low melt solder forms the matrix betweenthe conducting graphite platelets. In such embodiments, a paste isformed with appropriate graphite powder and molten solder. The paste isapplied between the heat source and sink and heated above its meltingtemperature during the assembly process. After assembly the solder pastesolidifies upon cooling and retains the three-dimensional shape of thegap. This is very important because chip modules are often highlynon-planar because of module camber. In addition to camber on singlechips, multichip modules contain numerous chips that are preferablycooled with a single solution. The thermal spreaders must be attached tochips of varying heights, making an exact fit between the chip and hatdifficult. Since the gap can adopt a non-predictable shape because ofthe camber or variations in chip height, it can only be effectivelyfilled by either a paste that remains fluid during use or by a solidthat was molded in place for that individual module.

In yet a further embodiment, graphite particles are dispersed in avolatile liquid to create a paste. Such a paste is delivered to the gapbetween the chip and the heatsirik by normal paste dispense methods. Thevolatile liquid media (solvent) is then evaporated under vacuum and/orheat leaving behind a compacted collection of the particles which havebeen formed into the exact shape of the gap including allnon-uniformities arising from module camber. This now-dry compactedcollection of graphite (for example) microplatelets could then becontacted with a low melt solder. The solder would wick into the powder,and upon cooling solidify the composite mass which would be highlythermally conductive and perfectly conformed to the shape of the gap.

An experiment was carried out to demonstrate that platelets would indeedself orient with the graphitic plane parallel to the desired directionof heat flow (perpendicular to the chip and heat sink plane). A singlecrystal graphite disk with a 1.5 inch diameter was sawed into a diskonly 10 mils thick where the basil plane was perpendicular to the planeof the disk. This was then diced in a chip dicing tool into rectanglesno smaller than 1.5 mm in dimension. A collection of these rectangular(mostly square) pieces were then mixed randomly in an epoxy glue andallowed to settle as the glue was hardening. FIG. 10 demonstrates thatthe disks indeed laid flat and stacked as desired proving the concept ofself orientation. The figure shows a cross section of graphite blocks250 um thick by 1 to 5 mm long and wide. The graphite basal plane isoriented in the z-direction. These blocks were mixed randomly into epoxyand allowed to settle. Very close and parallel stacking is observed,leading to effective z-direction thermal conductivity through the stack.

The present invention is applicable to materials other than graphite.For instance, there would be a thermal conductivity advantage tomicrotome slicing of very small diameter copper wire. This would resultin disk shaped particles that would stack providing a dramaticallyincreased surface area (plane to plane) contact from one particle to thenext relative to the point to point contact that jagged randomly shapedcopper particles have. Copper wire can be purchased in very fine (1 mil)and smaller diameters. Microtoming could be performed on large bundlesmaking the fabrication of bulk quantities of copper disks feasible.While copper is not isotropic, the disk shapes would ensure goodstacking in a paste application.

1. A thermally conductive paste composition comprising: platelets of athermally-conductive material, wherein said platelets have an aspectratio of at least 2:1; and a liquid.
 2. The thermally conductive pastecomposition, according to claim 1, wherein said thermally-conductivematerial comprises a metal.
 3. The paste composition of claim 1, whereinsaid liquid is selected from the group consisting of liquid metals andoils.
 4. The paste composition of claim 3, wherein said oil is apolyalphaolefin oil.